Described below are a method and a device for determining the contact angle between a sample surface, a gaseous environment and a drop arranged on the sample surface, the drop having a curved surface, a symmetry axis and a defined volume.
Here, the contact angle is the angle between the tangent to the contour of a drop and the straight line through the three-phase point of the drop on the sample surface, a drop of a known liquid being studied on a sample surface to be characterized. This angle is influenced inter alia by the surface tension of the drop, the surface energy of the sample surface being considered and the interfacial interaction between the drop and the sample surface. By determining the contact angle for drops of different liquids with known surface tensions, it is possible to deduce the surface energy of the sample.
There are various optical methods for contact angle determination. On the one hand, for example as in DE 197 54 765 C1 or KR 102 002 007 479 7 A, a shadow image method is employed in which the drop lying on a sample is observed laterally by a camera and the contact angle is determined with the aid of the two-dimensional recording of the drop contour and subsequent suitable image processing. The disadvantage of this drop-contour or shadow-image method is that the drop to be observed must be fully recorded by the camera.
On the other hand, the contact angle can be determined by a direct viewing method. Optics arranged above the sample, as well as a light source, are used here to record an image of the drop. The drop on a sample surface is illuminated with a direct light, so that the drop diameter can be determined by image analysis. With a known drop volume, the contact angle can be determined with the aid of the diameter of the drop.
To this end, it is known from JP 60 085 353 A to image a drop by a direct viewing method and, with the aid of the recording, to determine the diameter of the contact surface of the drop with the sample and the height of the drop. These two values are used to calculate the contact angle. WO 03/073045 A1 furthermore discloses a method in which a drop on a sample surface is recorded optically by a camera arranged above it. The bearing surface of the drop is subsequently calculated by a mathematical approximation of the drop surface to a circular surface.
Even in the contact angle determination methods carried out by the previously known direct viewing devices, it is necessary to image the drop fully so that the determination of a contact angle for a sample surface which has a very large area or is difficult to access, for example lying in a cavity, is not possible or is possible only with great outlay. The measurement of contact angles which are small, i.e. on drops which are significantly flattened, is also frequently not possible since these drops often cannot be recorded fully by the field of view of the camera optics owing to their large diameter.
A further disadvantage of the known methods with a direct viewing device is the generally low-contrast image relative to the sample surface, due to the transparency of the drop. This makes image analysis more difficult.